Fix gallery link option

~~~
Galleries should support three link types:

* Attachment Page (default): [gallery ids="1,2,3"]
* Media File: [gallery link="file" ids="1,2,3"]
* None: [gallery link="none" ids="1,2,3"]
~~~

Only thing I'm not sure about are the classes @ [[L93:C80]](https://github.com/leoj3n/roots/blob/261fb43776d8bdad7275d2e23d0e55e49cba2109/lib/gallery.php#L93)